Hydraulic system



Jan; 7, 1958 F. J. WRIGHT v HYDRAULIC SYSTEM Filed April 15. 1953 2Sheets-Sheet 1 .51 x K w m. m H m E .IIJWS m J D 1 mm M my flu; P2 B NNNa ms 2 0 KL \M N: @3 W3 PS 3N on N- f mu. /& m-

Jan. 7, 1958 F. J. WRIGHT 5 HYDRAULIC sys'rm 7 Filed April 15, 1953 v I2 Sheets-Sheet 2 I I Maw W6, I a I N it Q 88 INVENTOR; 7 n I I By FEED T13 g rh-TY 2,818,706 HYDRAULIC SYSTEM Fred J. Wright, Columbus, Ohio,assiguor to The Jeffrey Manufacturing Company, a corporation of OhioApplication April 15, 1953, Serial No; 348,983 2 Claims. (CI. 60-53)This invention relates to hydraulic apparatus and systems.

It is an object of the invention to provide improved hydraulic systemswherein there is a substantially closed hydraulic circuit having highpressure and high volume pump means and hydraulic motor means driventhereby, wherein hydraulic fluid is recirculated and wherein provisionis made for minimizing cavitation in the hydraulic circuit caused by anysudden displacement of hydraulic fluid from the low pressure side of thecircuit.

Other objects of the invention will appear hereinafter, the novelfeatures and combinations being set forth in the appended claims.

In the accompanying drawings:

Fig. l is a diagrammatic showing of an improved bydraulic apparatus andsystem including the invention;

Fig. 2 is a diagrammatic showing of another hydraulic apparatus andsystem including the invention;

Fig. 3 is a diagrammatic showing of still another hy-.

draulic apparatus and system; and

Fig. 4 is a diagrammatic showing of the apparatus and system seen inFig. 3 as it is applied to drive the coal cutting elements of a miningmachine.

The hydraulic apparatus and systemshown in Fig. 1 of the drawingsincludes a closed high pressure and high volume hydraulic circuit whichincludes a high pressure variable delivery pump which is capable ofbeing adjusted from a zero output volume to a high output volume ofhydraulic fluid under high pressure. The high pressure and high volumehydraulic circuit also includes a hydraulic motor 11. Conduits 12 and 13connect the high pressure variable delivery pump 10 with the motor 11.The pump 10, is indicated in the drawings, is discharging or pumpinghydraulic fluid into the conduit 12 to drive the motor 11. Conduit 13conducts the hydraulic fluid exhausted from the motor 11 to the intakeor suction port of pump 10.

- The hydraulic fluid in the closed circuit, including pump. 10, highpressureconduit 12, motor 11, and low. pressure or return conduit 13, isrecirculated, that is, fluid exhaustedby the motor 11 flows back tothepump 10 throughthe low pressure or return conduit 13 and is pumped bythe pump 10 through the high pressure conduit 12 back to the motor 11.The output volume of pump 10 may be: varied from zero to a high volume,and the pump 10 is shown in the drawings as including a rotor 14 havingreciprocating vanes 15 that operate in a ring or housing 16, the axis ofwhich may be shifted with respect to the axis of the rotor 14. It. willbe seen, of course, that when the axis of the rotor 14 and the axis ofthe ring or housing 16 coincide, the pump 10 will not be operating toproduce any output of hydraulic fluid into the conduit 12. In order tocause the pump 10 to pump hydraulic fluid, the axis of ringor housing 16may be shifted or offset upwardly, as. seen in the drawings, withrespect to the axis of rotor 14, and the amount of oflsetting of theseaxes determines the output volume of pump 10. The ring or housing 16 isshifted upwardly, as seen in the drawings, to increase its output volumeby a piston motor 17, and it is shifted downwardly to decrease itsoutput volume by a piston motor 18. -Although Fig. l of the drawingsshows pump 10 as being a variable volume vane type pump, it is to beunderstood that any type pump having variable or fixed output featuresmay be substituted in the system for the pump 10.

- 26 may be of any well known construction and it may bev 2,818,706Patented Jan. 7, 1958.

The entire hydraulic system also includes a' tank 19 which is areservoir for hydraulic fluid used in the hydraulic system. From thehydraulic tank19 a 'supplyconduit 20 leads to a low volume, low pressureconstant delivery pump 21. The hydraulic fluid pumped by pump 21 isnormally conducted through a conduit 22' into one end: of a manuallyoperablecontrol valve-23f For the momentit will suflice to say of thecontrol valve 23' that hydraulic fluid normally passes through it and isdischarged from the opposite end thereof into a conduit24 that leads tothe low pressure of returnconduit 13 which connects the exhaust port ofmotor 11 and the intake or low pressure port of pump 10. Low pressure orreturn conduit 13 is connected through a conduit 25, a relatively lowpressure relief valve 26 and a conduit 27 to the tank 19. *Relief valvesure to hydraulic fluid forced thereinto is connected to the conduit 25leading to the pressure relief valve 26 and thus to the low pressureconduit 13 of the closed hydraulic circuit including pump 10, highpressure conduit 12, motor 1 1,'an'd low pressure or return conduit 13.

The manually operable control valve 23 is preferably identical to one ofthe valves shown and described in my Patent No. 2,486,087, dated October25, 1949,1for a hydraulic valve, but it may be of any other well knowndesign having at least three control positions, namely 'a' central, orneutralposition in which all of the hydraulic fluid being received by itfrom conduit 22 is directed into the conduit 24 and a position at eachside of said central.

or neutral position wherein all of the hydraulic fluid being received bythevalve through conduit 22 is directed to one or the other of a pair ofbranch conduits 29 and 30 and that conduit 29 or 30 not receivinghydraulic fluid from conduit 22 is connected to the conduit 24 bythevalve.

Branch conduit 29 conducts hydraulic fluid to motor 17 which shifts thering or housing 16 to increase the output volume of the main pump 10 andconduit 30 conducts hydraulic fluid tomotor 18 which shifts the ring orhousing 16 to decrease the output volume of main pump 10. From theforegoing description of manually operable valve 23 it will beseen thatwhen the hydraulic fluid is being directed, for example, to motor 17that a quantity of hydraulic fluid equal to that supplied to motor17jwill be expelled from motor 18 into conduit 24 and that thus thevolume of hydraulic fluid flowing through conduit 24 low'pressure orreturn conduit 13 will remain substantiallyconstant regardless of themanual operation of valve 23. It will also be seen that when valve 23 isin its central, neutral or by-passing position both branch conduits 29and 30 are blocked whereby motors 1'7 and 18 will retain. ring orhousing 16 of main pump 10 in that position which it occupied when valve23 was shifted to'its central, neutral or by-passing position.

in order that the output volume of hydraulic fluid from pump 21 will notbe blocked should eithermotor 17 or 13 reach the limit of its travel, aconduit 31 including a pressure relief valve 32, is connectedbetweenconduits 22 and 13. v In an apparatus including a system of the abovedescribed type the maximum output pressureof .pump 10 was in the rangeof 2000 tov 3000 p. s. i, and its maximum output volume was in the rangeof 70-80 G. P. .M. and low pressure, low volume pump 21 operated atanoutput pressure in the range of 50-150 p. s-..i. and output volume ofapproximately 8-10 G. P. M. a

Relief valve'26 is setor adjusted to open within a relative low pressurerange, for example 50-150' s. i., which, of course, determines thepressure in the low pres sure or return conduit 13, and relief valve 32is adjusted to open at a pressure slightly greater than that at whichvalve 26 opens. Relief valve 32 determines the maximum pressure ofhydraulicfluid that may beernployed to operate motors 17 andlS.

The above described hydraulic apparatus may be employed,-for example, todrive a coal cutting element'of a miningmachine and when this apparatusis operating to drive said'cutting element the pump circulates hydraulicfluid through the closed circuit including high pressure conduit'1 2,motor 11, and low pressure or return conduit 13. A volume'of hydraulicfluid equal to the output volume of pump 21 is at all times being forcedinto the conduit 13 through the conduit 24 or 31 and consequently thevolume of hydraulic fluid substantially equal to the output volume ofpump 21 must be displaced from the closed circuit. This hydraulic fluidis displaced from the closed circuit through the conduit 25, reliefvalve-26' and conduit 27 to tank 19. Relief valve 26 will, of course,open at a predetermined pressure and consequently the accumulator 28connected to conduit 25 will at all times be subjected to that pressureat which relief valve 26 is set to open;

The system above described, with the exception of the accumulator 28, isthe basic system of the reversible system described in my Patent No.2,621,479, dated December 16, 1952, and, of course it will be readilyapparent to those skilled in the art that the invention herein describedmay be applied to the system of said Patent No. 2,621,479 and be withinthe spirit and scope of certain of the appended claims.

It has been found that when the present system is employed, for example,to drive a coal cutting element of a mining machine that when the coal'cutting element is' loaded substantially constantly the system describedoperates efficiently. However, it has also been found that when, for anyreason, the load on the cutting element is relieved suddenly and theload is again suddenly placed upon the coal cuttingelement which, forexample, will occur if the'coal cutting element should chatter whileworking; parts, such as the niotor 11 and pump 10, may be damaged byshocks delivered to them through the hydraulic'systeni. To'prevent thesedamaging elfects and consequent destruction of expensive elements of theapparatus presented a real and serious problem in' connectionwithhydraulically driven elements such as'the coal cutting" elements abovereferred to.

Insofar as the above described hydraulic apparatus is concerned, itis-the application of the hydaulic accumulator 28in the hydraulic systemthat is important. By means of the accumulator 28 it is possible toemploy the apparatus described to drive a working element, such as a-coal' cuttin'g'element, and to minimize the destructive action ofjerking or chattering of the driven working or cutting element upon theparts of 'the' hydraulic apparatu's; It appears t'obe that inthe'describe'd apparatus, not including the accumulator 28',when'the'w'orking'load or resistance met by the driven working elementdriven by motor 11 is suddenly relieved, th'ere' occurs what may betermed" an elongation' of the column of hydraulic fiuidin the highpressure conduit 12' which may be due to'the expansion and' contractionof the mechanical elements that comprise the high pressure side of theclosed circuit, such, for example, as contraction of'the piping thatforms the conduit 12, and expansion, etc, of parts of 'both' the pump 10and motor 11. This so called elongation of the column'of hydraulic fluidin the high pressure" side of the closed hydraulic system causes themotor 1'1 to run momentarily at a speed greater than that at which itwas being driven under load, and consequently during this momentaryperio'd'r'riotor' 1'1" exhausts more bydraulicfluid into conduit 13 thancan be accepted by pump 10. This excess of hydraulic fluid is, ofcourse, discharged from the low pressure side of the closed hydraulicsystem through the relief valve 26. In other Words, the high pressure side, of the closed circuit may be compared with an accumulator and itmay be said to function as such.-

It will be seen, of'course, that when this action, herein termedelongation, occurs in' the high pressure side of the closed hydraulicsystem, the pump 10, lagging behind the sudden discharge ofhydraulicfluid from motor 11, will be required to withdraw s'uddenlya slightlylarger quantity of hydraulic fluid from the conduit 13 in the low sideof the closed hydraulic system and because the pump 21 cannot supplyhydraulic fluid to the low pressure side of the closed system with therapidity required, cavitation may occur in the "hydraulic systemaccompanied by pounding or jarring that results in damage and oftencomplete destruction of elements, such as the pump 10 and motor 11 ofthe hydraulic apparatus.

1 have found that by placing a hydraulic accumulator, such asaccumulator 28, in the low pressure side of the closed hydraulic systemthat the above described pounding or jarring may be minimized. When anaccumulator,

such as accumulator 28, is employed in the hydraulic apparatus, as abovedescribed, it appears that the accumulator 28 will displace hydraulicfluid under pressure from itself to make up in the low pressure side,and specifically in conduit 13, a quantity of hydraulic fluidsubstantially equal to that quantity of hydraulic fluid which wasdischarged from the low pressure side of the hydraulic system throughmotor 11 when the load thereon was relieved suddenly. This action ofaccumulator 23, of course, maintains conduit 13 completely filled withhydraulic fluid at all times and it tends to balance the accumulatoreffect of the high pressure side of the closed circuit, therebyeliminating the above described detrimental cavitation in the hydraulicsystem.

In Fig. 2 of the drawings there is shown diagrammatically a portion of ahydraulic apparatus and system that includes two high pressure, highvolume pumps and 1-10 driven from separate electric motors 114 and 114',respectively. These pumps may, if desired, be driven from a singleelectric motor. The apparatus ineludes two high pressure, high volumehydraulic motors 111 and 111 driven by hydraulic fluid discharged fromhigh pressure, high volume pumps 110 and 110'. This hydraulic systemincludes a portion of the system described in connection with Fig. 1 ofthe drawings, which portion is not duplicated in the drawings but whichis clearly described hereinafter, and it includes what is in effect twoclosed high pressure, high volume hydraulic circuits, one includingpumps 110 and motor 111, and the other including pump 110' and motor111'. Each of these closed circuits correspond for the most part withthe closed circuit including pump 10 and motor 11 described inconnection with the system shown in Fig. l of the drawings. The closedhydraulic circuit including pump 110 and motor 111 will now bedescribed, it being understood that the closed hydraulic circuitincluding pump 110 and motor 111' is a substantial duplicate of thedescribed circuit and that the corresponding parts or elements of thiscircuit carry numbers that correspond with the numbers of the describedcircuit, except that they carry an exponent. I

Conduits 112 and 113 connect the pump 110 with motor 111, and pump I10discharges or pumps hydraulic fluid into the high pressure conduit 112to drive motor 111. Low pressure or return conduit 113 conductshydraulic fluid exhausted from the motor 111 to the intake or suctionport of pump 110. Low pressure or return conduit 113 is connectedthrough a conduit to a relief valve 126 which corresponds with and hasthe identical function of relief valve 26. Relief valve 126 is connec'ted to the tank 113 through" a conduit 127, Thefluid is taken fromthe tank 119 by the punip'121 through the line 120 and fed through line122 to the hydraulic cir-' or return conduit 113; 'Accum'ulator'128 hasthe' same function in this system asaccumulator 28 has in the system ofFig. 1, and that portion of the circuit thus far described is capable offunctioning as do'es'the high pressure, high volume circuit previouslydescribed in connection with thesystem of Fig. 1 of the drawings.

.It is pointed out that the motors 111 and 111' are not interconnectedso that the driving effort of one may aid the other and that thereforeeach motor 111 and 111' may be employedto drive separate workingelements, such as coal cutting elements or heads of a mining machine.Because the motors 111 and 111' are not interconnected the workingpressures in the high pressure sides, namely conduits 112 and 11 2, ofthe closed circuits for the two. motors may differ in accordance withthe loads placed separately upon the motors 111 and 111- In this systemlow pressure conduits 113 and 113 are interconnected through the conduit125. The entire system includes two accumulators 128 and 128', eachconnected directly through connector conduits 134 and 134' to lowpressure or return conduits 113 and 113', respectively, whereby theiraction in supplying hydraulic fluid to conduits 113 and 113 will beimmediate.

It may be pointed out here that the pumps. 110 and 110 are shown as ofthe constant volume output type and that therefore operating oradjusting motors, such as motors 17 and 18, are not shown in associationwith pumps 110 and 110. Pumps 110 and 110' may be of the variabledelivery type, such as shown in Fig. 1 of the drawings, if desired. I

The hydraulic circuit including pump 110 and motor 111 differs from thesystem including pump and motor 11 in another respect. Referring againto Fig. 2 of the drawings, it will be seen that a relief valve 135 isinterposed in-the high pressure conduit 112. The structure and operationof rcliefvalve 135 is well known and it follows the structure of therelief valve shown and de scribed by G. C. Bonnell in his United StatesPatentNo. 2,388,820, dated November 13, 1945, and only so much of valve135 is here described as is necessary to an understanding of the presentinvention.

Relief valve 135 includes a body or housing 136 having a through passage137 which may be considered as a portion of conduit 112 through whichhydraulic fluid flows'from pump 110 to motor 111. Body 136 has acylinder in which a'valve core 138 reciprocates. Core 138 includes apiston or head 139 that slides in the above mentioned cylinder, and ahollow stem-140 that closes an exhaust port 141 to prevent fluid in thethrough passage 137 from flowing from the'valve body 136 through theexhaust port 141. One side of piston 139 is exposed to the pressure ofthe hydraulic fluid in through passage 137, and the opposite side ofpiston-.139'is exposed to hydraulic fluidin a chamber 142. Thus thepressure of fluid in the chamber 142 on one side of piston 139 opposesthe pressure of fluid on the other side of the piston 139 and in thethrough passage 137. Piston 139 includes a small bore 143 thatinterconnects the chamber 142 with the through passage 137. The body ofvalve 135 includes a head 144 in which there is a passageway thatleadsfrom the chamber 142 to a ball check valve that may be adjusted toopen at a desired pressure by a spring and? adjusting screw 145.Hydraulic fluid which flows past this check valve is discharged throughthe hollow amma and out of-thevalve-through the exhaust port 141.Valve'core-138 is'urged in a direction to close port bore 143. When thepressures in the chamber 142, and

through passage 137 are balanced, valve core 138 is urged to closeexhaust port 141 by the spring in the hollow stem. It will be seen thatif hydraulic'fluid is removed from chamber 142 at a rate "greater thanthe capacity of bore 143 topermit'fl'uid frorn'through passage 137 toreplace it, the pressure of hydraulidfluid'in through passage 137 willmove valve core 138 and open the exhaust port 141, or if the pressure inthrough 'passage 137 should be increased suddenly the same function willoccur because hydraulic fluid will be exhausted from chamber 142 throughthe ball check valve and hollow stern 140, and piston 139 will movehollow stem 140 to open port 141.

In the hydraulic system being described the exhaust port 141 of reliefvalve 135 is connected with the low pressure conduit 113 by a conduit146 so that hydraulic fluid discharged from conduit 112 through reliefvalve 135 is conducted to low pressure conduit 113. It maybe mentionedhere that sudden operation of relief valve 135 may cause the sameelongation effect in the hydraulic system as is caused when a load isremoved suddenly from the motor 111 or 111 as above described.

The head 144 of relief valve 135 also includes a passageway 147connected with chamber 142. This passageway is connected through aconduit 148 including a check valve 149 that permits hydraulicfluid toflow only out of chamber 142 to a manually controlled venting valve 150that is connected to exhaust to tank 119 through a conduit 151. It willbe seen'that if venting valve 151 is opened relief valve 135 will .beopened to cause hydraulic fluid in conduit 112 to by-pass the motor 111through valve 135 and conduit 146 directly into con-' duit 113, thussevering the driving connection between the pump and motor 111. Ventingvalve 150'constitutes a control by which the motor 111 may beeffectively and quickly disconnected from the driving effort of the pump11without stopping the pump. Obviously, when venting valve 150 is openedrelief valves and 135 will be operated to disconnect bothmotors 111 and111' from the driving effort of their respective pump 116 and 110.

Figs. 3 and 4 illustrate a modified hydraulic apparatus and system. Fig.3 illustrates the apparatus and' system diagramatically, and Fig. 4illustrates this apparatus and system as it has actually been employedto drive the coal cutting elements of a mining machine.

Referring first to Fig. 3 of thedrawings, the system differs from thesystem of Fig. 2 above described, in the followingrespects. There isonly one accumulators 228 which has the function in this system'of bothof the accumulators 128 and 128' of the system of Fig. 2. Relief valves235 and 235 which occupy positions in the system corresponding withrelief valves 135 and 135' of Fig- 2 are modified, motors 211 and211'1are mechanically interconnected through gearing indicated by thenumeral 252 to operate together, and pressure conduits 212 and 212 areinterconnected through a conduit 233. All of the remaining parts of thissystem correspond with the parts of the system of Fig. 2 and they areassigned numbers in the two hundred series, whereas the parts of thesystem of Fig. 2 are assigned numbersin the one hundred series. Exceptfor the differences ,herein specifically set forth, the description ofthe system of Fig. 2 is an ac-. curate'description of the system of Fig.3. V

, Relief, valves 235 and 235 are modified to the extent that theirHeads-244' do not include the ball check valve d-by screyv 145, andtherefore these valves W any in response to pressure in the all h hprssureconduits 212 and 212', but respond simultano'usly to the openingof a fixed or manuallyff'djustahle pressure relief valve 262. Valve 262dethe maximum working pressure of hydraulic nl'thepre'ssure" conduits212 and 212. This system b ineludes a fmanua'lly controlled ventingvalve 250 wt H has the identical function" in this system as does the veting valve 150 in the ystem of Fi .2.

' eeausen otors 211 and 211' are mechanically interconnected,'thissystemwill operate efficiently with only one accumulator 228 and this becausemotors 211 and 2111"msyescn arathe driving effort of the other andbecause it is; unlikely that the load will ever be relievedsuddrily'on'bothirnotors 211 and 211' simultaneously, and ply rarelywill the accumulator 228 be required to discharge into both low pressureor return conduits 2 13" and"213" simultaneously although it is capableof doing so through the conduit 225. In this system the conduit 22 5causes the hydraulic pressures in low pres.- sure or return conduits 213 and 213 to balance, and conduit 23'3fcauses the hydraulic pressuresin pressure conduits '212' and 212 to balance.

In this last described hydraulic system as applied to one mining machineof which parts are illustrated in Fig f4'of the drawings, each main pump110, 110' has an" output capacity of '70 G. P. M. and it operates at amaximum pressure of 2000 p. s. i. as controlled by the setting "of valve262. The low pressure, low volume pump 221 'hasaca'pacity' of 18 G. P.M. and the relief valve 250 is setto open at 125 p. s. i.

It'is ito be' understo'od that all of the pressures in the systems-aboveset forth are exemplary of high pressure and high volume circuits inwhich the present invention may be'ernployed with great advantage, butthat in other and broader aspects of the invention it may be employed inclosed hydraulic systems or circuits wherein the pressuresdiffe'r widelyfrom the examples set forth.

As mentioned above,- Fig. 4 illustrates the systemshown in Fig.3 appliedto a coal mining machine to drive the coal cutting elements thereof. Thecoal cutting elements illustrated in Fig. 4 consist of an upper row ofrevolvable arms'253 each of which is mounted on the front end of a shaft254 carri'ed by the housing of an upper cutter head 255. Shafts 254 liein side by side relation and in a generally horizontal plane and, asindicated in the drawings, are. driven through an obvious gear train sothat-some of the shafts 254 rotate in one direction and the othersrotate in reverse direction. Head 255 includes a reductiongearingindicated at 256 for which the gear train for shafts 254, andconsequently arms 253, is driven by hydraulic motor 211.

Situated directly. below the upper cutter head is a second cutter head257 which, as shown in the drawings, is a substantial duplicate, thoughof somewhat dilferent arrangement, of cutter head 255 and which drives aless number of cutter arms 258. Lower cutter head 257 is driven byhydraulic motor 211' through reduction gearing259 similar to thatindicated at 256 in the upper cutterhead 255. The reduction gear trains256 and 259 are interconnected in a manner that is obvious from thedrawings'by 'the'gean'ng 252, whereby cutter heads 255 and, 257 aredriven simultaneously by motors 211 and 2 11 In Fig. 4 the low pressurelow volume pump 221 is ,shown as being driven by a separate motor 260through a flexible coupling 261, but it is to be understood that pump 22l may be driven ifdesired in any suitable mannerlby either of themotors214 or 214.

, Obviously those skilled in the art may make various changes in thedetails and arrangement of parts without departiiig frorn the spirit andscope of the invention as defined by the claims hereto appended, andapplicant therefore wishes not to be festricted to the precise cononrein" unb ased.

of saidmotors and' the other' connecting the highprs-I sure'output prter th'efotherf f said umps with the inlet port of the other of saidmotors, means forming two conduits for low pressure hydraulic fluid,'oi'le connecting the low pressure'intake port of one of said pumps-withthe exhaust port" ofone etfisaidjmotors and the other connectingthe low'pres'sure intake'po'rt of the other of said pumps with theiihaust port of the other of said motors, by-p'a'ss conduitmfeans'inclnding a relief valve interconn'ecting 'said' high and lowpressure conduithians connected to ea'ch of' said motors, conduit meansconnecting said pair of'high pressure conduits, conduit means connectingsaid pair ofjlow pressure conduits,'said last named two conduit meansfunctioning to' cause the hydraulic pressures insaidhigh pressureconduits to remain equal and the pressures in'said low pressure conduitsto remain equahineans supplying fluid" to said pair of low pressureconduits, and accumulator means connected to said low pressure conduitsfor adding fluid thereto'to replace fluid displaced from 'said circuitas a result of sudden increases'in the output" volume of said motorsthereby preventing cavitation'in'said circuit. I

2. A hydraulic circuitfor driving two hydraulic motors mechanicallyinterconnected to operate together, said circuit including' twohydraulic pumps and two hydraulic motors, means forming two conduits'for high pressure hydraulic fluid,"oneconnecting the high pressureoutput port ofone of said pumps with the inlet port of one c-fsaidmotors afnd the other connecting the high pressu'reg out'put port ofthe' other of said pumps with the inlet portjof the other of saidmotors, means forming twofcon'dui'ts for low"pressure hydraulic fluid,one conmeeting the low pressure intake port of one of said pumps withthe eiihaustpo'rt of one of said motors and the other 'conne'cti ng thelow pressure intake port of the other of said pumps with the exhaustport of the other of 'said motors, by-pass" conduit'means including arelief valve interconnecting said high and low pressure conduit meansconnected to each' of said motors, conduit means connecting"'said pairof high pressure conduits, conduit means connecting'said 'pair of lowpressure conduits, said last namedtwo conduit means functioning to causethe hydraulic pressures in 'saidhigh pressure conduits to remain equaland the pressures in said low pressure conduits to remain equal, meanssupplying fluid to said pair of low-"pressure conduits, and accumulatormeans connected'to 'said'low pressure conduits for adding fluid theretoto replace fluiddisplaced from said circuit as a result of suddenincreases in the volume of fluid passing through said last named reliefvalve means thereby preventing cavitation in said circuit.

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